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United States Patent |
5,251,604
|
Kaneko
,   et al.
|
October 12, 1993
|
System and method for detecting deterioration of oxygen sensor used in
feedback type air-fuel ratio control system of internal combustion
engine
Abstract
A feedback type air-fuel ratio control system controls the air-fuel ratio
of air-fuel mixture fed to an internal combustion engine in accordance
with an information signal issued from a first oxygen sensor installed in
an exhaust line of the engine. The exhaust line has a catalytic converter
at a position downstream of the first oxygen sensor. There is further
provided a system in the control system, which detects deterioration of
the first oxygen sensor. The system comprises a computer and a second
oxygen sensor of delayed response type installed in the exhaust line at a
position upstream of the converter. The computer defines higher and lower
slice levels with respect to the output of the second oxygen sensor and
compares the output of the second oxygen sensor with the higher and lower
slice levels.
Inventors:
|
Kaneko; Hiroaki (Kanagawa, JP);
Nemoto; Kouichi (Kanagawa, JP);
Okada; Akihide (Kanagawa, JP)
|
Assignee:
|
Nissan Motor Company, Ltd. (JP)
|
Appl. No.:
|
713230 |
Filed:
|
June 11, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
123/688; 123/691; 123/695 |
Intern'l Class: |
F02D 041/14 |
Field of Search: |
123/688,690,691,694,695
|
References Cited
U.S. Patent Documents
4117815 | Oct., 1978 | Ikeura | 123/691.
|
4337746 | Jul., 1982 | Masaki | 123/691.
|
4854288 | Aug., 1989 | Uchikawa | 123/691.
|
4915080 | Apr., 1990 | Nakaniwa et al. | 123/691.
|
5025767 | Jun., 1991 | Kume et al. | 123/691.
|
5065728 | Nov., 1991 | Nakaniwa | 123/694.
|
5095878 | Mar., 1992 | Kumagai et al. | 123/691.
|
Foreign Patent Documents |
58-47248 | Mar., 1983 | JP.
| |
59-215935 | Dec., 1984 | JP.
| |
139943 | Jun., 1987 | JP | 123/691.
|
Primary Examiner: Wolfe; Willis R.
Attorney, Agent or Firm: Lowe, Price, LeBlanc & Becker
Claims
What is claimed is:
1. In a feedback type air-fuel ratio control system which controls the
air-fuel ratio of air-fuel mixture fed to an internal combustion engine in
accordance with an information signal issued from a first oxygen sensor
installed in an exhaust line of said engine, said exhaust line having a
catalytic converter mounted thereto at a position downstream of said first
oxygen sensor,
a combination which comprises:
a second oxygen sensor installed in said exhaust line at a position
upstream of said catalytic converter, said second oxygen sensor being of a
delayed response type;
first means for defining higher and lower slice levels with respect to the
output of said second oxygen sensor; and
second means for detecting deterioration of said first oxygen sensor by
comparing the output of said second oxygen sensor with said higher and
lower slice levels.
2. A system as claimed in claim 1, in which said higher and lower slice
levels are positioned at higher and lower output sides of said second
oxygen sensor with respect to an output of the sensor which is produced
when the air-fuel mixture has a stoichiometric air-fuel ratio.
3. A system as claimed in claim 2, in which said second means comprises:
means for reading maximum and minimum levels of the output of said second
oxygen sensor; and
means for judging the condition of said first oxygen sensor by finding one
of five states,
said five states being:
(a) a state wherein said maximum level is higher than said higher slice
level and said minimum level is lower than said lower slice level;
(b) a state wherein said maximum level is lower than said lower slice
level;
(c) a state wherein said maximum level is lower than said higher slice
level and higher than said lower slice level;
(d) a state wherein said minimum level is higher than said higher slice
level; and
(e) a state wherein said minimum level is lower than said higher slice
level and higher said lower slice level.
4. A system as claimed in claim 3, further comprising third means which
controls the air-fuel ratio of air-fuel mixture in accordance with an
information from said second means.
5. A system as claimed in claim 4, in which said third means enriches the
air-fuel mixture when said second means finds the states (b) and (c) and
leans the air-fuel mixture when said second means finds the states (d) and
(e).
6. A system as claimed in claim 5, in which said third means keeps the
existing air-fuel ratio of the mixture when said second means finds the
state (a).
7. A system as claimed in claim 1, in which said second oxygen sensor
comprises a base structure of zirconia, first and second platinum
electrodes lines on respective surfaces of said base structure, a spinel
layer lined on said second electrode and a catalyst layer lined on said
spinel layer, said catalyst layer including noble metals and ceria.
8. In a feedback type air-fuel ratio control system which controls the
air-fuel ratio of air-fuel mixture fed to an internal combustion engine in
accordance with an information signal issued from a first oxygen sensor
installed in an exhaust line of said engine, said exhaust line having a
catalytic converter mounted thereto at a position downstream of said first
oxygen sensor,
method of detecting deterioration of said first oxygen sensor, which
comprises by steps:
monitoring the air-fuel ratio of the mixture by receiving an information
signal from a second oxygen sensor installed in said exhaust line at a
position upstream of said catalytic converter, said second oxygen sensor
being of a delayed response type;
defining higher and lower slice levels with respect to the output of said
second oxygen sensor; and
comparing the output of said second oxygen sensor with said higher and
lower slice levels.
9. A method as claimed in claim 8, further comprising:
controlling the air-fuel ratio of the mixture in accordance with a result
of the comparison of said output with said higher and lower slice levels.
10. A feedback type air-fuel ratio control system of an internal combustion
engine which is equipped with a catalytic converter at an exhaust line,
said system comprising:
a first oxygen sensor installed in the exhaust line at a position upstream
of said catalytic converter;
control means for controlling the air-fuel ratio of air-fuel mixture fed to
the engine in accordance with an information signal issued from said first
oxygen sensor;
a second oxygen sensor installed in said exhaust line at a position
upstream of said catalytic converter, said second oxygen sensor being of a
delayed response type;
first means for defining higher and lower slice levels with respect to the
output of said second oxygen sensor;
second means for detecting deterioration of said first oxygen sensor by
comparing the output of said second oxygen sensor with said higher and
lower slice levels; and
third means for modifying said information signal of said first oxygen
sensor in accordance with an information from said second means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system and a method for detecting
deterioration of an oxygen sensor, and more particularly, to a system and
a method for detecting deterioration of an oxygen sensor used in a
feedback type air-fuel ratio control system of an internal combustion
engine.
2. Description of the Prior Art
Japanese Patent First Provisional Publications Nos. 58-47248 and 59-215935
show conventional feedback type air-fuel ratio control systems for an
internal combustion engine. In these systems, a basic fuel injection
quantity is calculated based on various engine operation informations,
such as, intake air amount, engine speed and the like, and the basic fuel
injection quantity is corrected in accordance with an information signal
issued from an oxygen sensor installed in an exhaust gas conduit. The
amount of fuel practically fed to the engine is controlled in accordance
with the corrected fuel injection quantity. This control is repeated in a
feedback manner for keeping the air-fuel ratio of air-fuel mixture within
a desirable or stoichiometric level.
FIG. 16 shows a block diagram of the conventional air-fuel ratio control
system of feedback type.
Designated by reference "A" is a computer-installed control unit. A basic
pulse is produced from signals respectively issued from an air-flow meter
and an ignition coil. The basic pulse is weighted by an information signal
from a throttle valve switch and corrected in voltage by a battery.
Furthermore, the pulse is weighted, through an air-fuel ratio feedback
control circuit "B", by an information signal issued from an oxygen sensor
1 and weighted by information signals respectively issued from a start
switch and a coolant temperature sensor. The corrected signal is then
treated by an arithmetic circuit and amplified by a power amplifier to
practically actuate fuel injectors.
FIG. 17 shows the detail of the air-fuel ratio feedback control circuit "B"
shown in FIG. 16. When the electromotive force of the oxygen sensor 1 is
higher than a reference voltage, it is judged that the air-fuel mixture
practically fed to the engine is richer than stoichiometric. Upon this,
the circuit "B" adds a so-called "mixture leaning signal" to the basic
signal for controlling the fuel injectors to inject smaller amount of
fuel. While, when the electromotive force is lower than the reference
voltage, it is judged that the air-fuel mixture fed to the engine is
leaner than stoichiometric. Upon this, the circuit "B" adds a so-called
"mixture enrichment signal" to the basic signal for controlling the fuel
injectors to inject larger amount of fuel. In fact, the amount of fuel
injected by the fuel injectors is controlled by varying the time during
which the injectors are opened.
FIG. 18 is a chart showing an output signal V' of the oxygen sensor 1 on
the axis of ordinates and elapsed time on the axis of abscissas. FIG. 19
is a graph showing an air-fuel ratio correction factor (.alpha.) on the
axis of the ordinates and elapsed time on the axis of abscissas. It is to
be noted that proportional factors PR and PL and integral factors IR and
IL shown in FIG. 19, which are the correction factors, are all constant.
Under the above-mentioned control, a catalytic converter (particularly,
three-way type catalytic converter) can exhibit high performance in
purifying the exhaust gas from the engine. However, hitherto, the
above-mentioned conventional air-fuel ratio control system has been
constructed without taking a severe consideration on deterioration of the
oxygen sensor which appears with passing of time. In fact, the output
characteristic of the oxygen sensor changes with the lapse of time. Thus,
after prolonged usage of the system, the stoichiometrically controlled
feeding of air-fuel mixture to the engine becomes out of order due to the
deterioration of the oxygen sensor and thus the exhaust gas from the
engine fails to have an exhaust composition which is suitable for allowing
the catalytic converter to exhibit its maximum performance.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a system and
a method for detecting deterioration of an oxygen sensor which is
operatively used in an air-fuel ratio control system of an internal
combustion engine.
According to a first aspect of the present invention, there is provided a
combination in a feedback type air-fuel ratio control system which
controls the air-fuel ratio of air-fuel mixture fed to an internal
combustion engine in accordance with an information signal issued from a
first oxygen sensor installed in an exhaust line of the engine. The
exhaust line has a catalytic converter mounted thereto at a position
downstream of the first oxygen sensor. The combination comprises a second
oxygen sensor installed in the exhaust line at a position upstream of the
catalytic converter, the second oxygen sensor being of a delayed response
type; first means for defining higher and lower slice levels with respect
to the output of the second oxygen sensor; and second means for detecting
deterioration of the first oxygen sensor by comparing the output of the
second oxygen sensor with the higher and lower slice levels.
According to a second aspect of the present invention, there is provided,
in a feedback type air-fuel ratio control system which controls the
air-fuel ratio of air-fuel mixture fed to an internal combustion engine in
accordance with an information signal issued from a first oxygen sensor
installed in an exhaust line of the engine at a position downstream of the
first oxygen sensor, a method for detecting deterioration of the first
oxygen sensor. The method comprises by steps monitoring the air-fuel ratio
of the mixture by receiving an information signal from a second oxygen
sensor installed in the exhaust line at a position upstream of the
catalytic converter, the second oxygen sensor being of a delayed response
type; defining higher and lower slice levels with respect to the output of
the second oxygen sensor; and comparing the output of the second oxygen
sensor with the higher and lower slice levels.
According to a third aspect of the present invention, there is provided a
feedback type air-fuel ratio control system of an internal combustion
engine which is equipped with a catalytic converter at an exhaust line.
The system comprises a first oxygen sensor installed in the exhaust line
at a position upstream of the catalytic converter; control means for
controlling the air-fuel ratio of air-fuel mixture fed to the engine in
accordance with an information signal issued from the first oxygen sensor;
a second oxygen sensor installed in the exhaust line at a position
upstream of the catalytic converter, the second oxygen sensor being of a
delayed response type; first means for defining higher and lower slice
levels with respect to the output of the second oxygen sensor; second
means for detecting deterioration of the first oxygen sensor by comparing
the output of the second oxygen sensor with the higher and lower slice
levels; and third means for modifying the information signal of the first
oxygen sensor in accordance with an information from the second means.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will become apparent
from the following description when taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a schematic block diagram of an air-fuel ratio control system of
an internal combustion engine to which the present invention is
practically applied;
FIG. 2 is a sectional view of an oxygen sensor which is used as a second
oxygen sensor in the present invention;
FIG. 3 is an enlarged view of the part enclosed by the circle "III" in FIG.
2;
FIGS. 4 to 6 are charts showing the output characteristics of first and
second oxygen sensors;
FIGS. 7 to 11 are charts showing output characteristics of the first and
second oxygen sensors in various conditions;
FIG. 12 is a block diagram of the air-fuel ratio control system to which
the present invention is practically applied;
FIG. 13 is a detailed view of a feedback control circuit employed in the
air-fuel ratio control system of FIG. 12;
FIG. 14 is a general flowchart showing the outline of operation steps
carried out in a control unit employed in the air-fuel ratio control
system;
FIG. 15 is a flowchart showing the detail of Step 4 of the general
flowchart of FIG. 14;
FIG. 16 is a view similar to FIG. 14, but showing a conventional air-fuel
ratio control system;
FIG. 17 is a view similar to FIG. 15, but showing a feedback control
circuit employed in the conventional control system of FIG. 16;
FIG. 18 is a chart showing a wave-form of output of an oxygen sensor; and
FIG. 19 is a chart showing an air-fuel ratio correction factor.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows, but schematically, an air-fuel ratio control system of an
internal combustion engine to which the present invention is practically
applied. Denoted by numeral 1 is a first oxygen sensor, 2 is a second
oxygen sensor, 3 is an internal combustion engine, 4 is a fuel injection
valve, 5 is an electronic controlling unit and 6 is a catalytic converter.
As the first oxygen sensor 1, commonly used sensors, such as, solid
electrolyte type, oxide semiconductor type, limiting current type and the
like are usable.
As the second oxygen sensor 2, such an oxygen sensor as shown in FIGS. 2
and 3 is used.
The sensor 2 comprises a conical structure 8 of zirconia, first and second
platinum electrodes 9a and 9b lined on inner and outer surfaces of the
conical zirconia structure 8, a spinel layer 10 lined on the second
electrode 9b and a catalyst layer 11 lined on the spinel layer. The
catalyst layer 11 includes noble metals and ceria.
With this catalyst layer 11, the second oxygen sensor 2 has a so-called
"delayed response characteristic".
During operation of the engine 3 (see FIG. 1), the first oxygen sensor 1
feeds the electronic controlling unit 5 with an information signal which
represents the oxygen concentration in the exhaust gas from the engine 3.
In accordance with the information signal, the controlling unit 5 controls
the fuel injection valve 4 to increase or decrease fuel injected
therefrom, so that the air-fuel mixture practically fed to the engine 3
has a desired or stoichiometric air-fuel ratio.
In accordance with the present invention, monitoring system is further
employed, which monitors whether the air-fuel mixture fed to the engine 3
is controlled within a stoichiometric level or not. Thus, the monitoring
system can detect the deterioration of the first oxygen sensor 1. That is,
the monitoring system is so designed as to judge whether an average
air-fuel ratio of the mixture is within the stoichiometric level or not.
The monitoring system includes the second oxygen sensor 2 and a comparative
means 7.
The output characteristic of the second oxygen sensor 2 is shown by a
phantom line in the charts of FIGS. 4 to 6. For comparison, the output
characteristic of the first oxygen sensor 1 is shown by a solid line in
the charts.
As is seen from FIG. 4, when the second oxygen sensor 2 is exposed to an
exhaust gas which is produced from an air-fuel mixture having a
stoichiometric or its neighboring air-fuel ratio, the output of the second
oxygen sensor 2 exhibits an output characteristic similar to that of the
first oxygen sensor 1.
However, as is seen from FIG. 5, when the exhaust gas is inclined toward a
richer side (that is, when the air-fuel mixture becomes richer), the
output of the second oxygen sensor 2 exhibits a richer characteristic, and
as is seen from FIG. 6, when the exhaust gas is inclined toward a leaner
side (that is, when the air-fuel mixture becomes leaner), the output of
the second oxygen sensor 2 exhibits a leaner characteristic.
This unique output characteristic of the sensor 2 is caused by presence of
ceria contained therein, as is described in Japanese Patent Application
No. 2-17910 filed by the same applicants.
It is to be noted that in the charts of FIGS. 4 to 6, ".lambda." is the
excessive air factor which is defined by dividing the quantity of air
supplied to the engine by the theoretical requirement, "V" is the output
curve of the second oxygen sensor 2, "V'" is the output curve of the first
oxygen sensor 1 and "C" is a control pattern.
As is shown in FIGS. 7 to 11, when the second oxygen sensor 2 having the
above-mentioned output characteristic is practically used, slice levels SR
and SL are provided at richer output side and leaner output side
respectively.
That is, when, as is seen from the chart of FIG. 9, the curve of the output
"V" of the second oxygen sensor 2 intersects both the slice levels SR and
SL, it is judged that the air-fuel ratio of the air-fuel mixture is
desirable. When, as is understood from the chart of FIG. 7, the curve of
the output "V" is located at a richer side and fails to intersect both the
slice levels "SR" and "SL", it is judged that the air-fuel ratio of the
mixture is richer than stoichiometric, and when, as is understood from the
chart of FIG. 8, the curve of the output "V" intersects only the slice
level "SR", it is judged that the air-fuel ratio of the mixture is
slightly richer than stoichiometric. When, as is seen from the chart of
FIG. 10, the curve of the output "V" intersects only the slice level "SL",
it is judged that the air-fuel ratio of the mixture is slightly leaner
than stoichiometric, and when, as is seen from the chart of FIG. 11, the
curve of the output "V" is located at the leaner side and fails to
intersect both the slice levels "SL" and "SR", it is judged that the
air-fuel ratio of the mixture is leaner than stoichiometric.
With this, it is possible to detect the deterioration of the first oxygen
sensor 1.
The practical air-fuel ratio control for the mixture will be described in
the following with reference to FIG. 12 which shows an air-fuel ratio
control system.
The system is substantially the same as the conventional system of FIG. 16
except the following.
That is, in the invention, the second oxygen sensor 2 and a comparator
means 7 which comprises an air-fuel ratio detecting circuit 7a and a
feedback factor correcting circuit 7b are added, and a modified feedback
control circuit "B'" is used as a substitute for the circuit "B".
During operation of the engine, the output of the second oxygen sensor 2 is
fed through the air-fuel ratio detecting circuit 7a to the feedback factor
correcting circuit 7b which serves as a so-called "air-fuel ratio
correcting means". At this means, the output of the second oxygen sensor 2
is measured for a given time under a certain engine operating condition
which is given by the engine condition detecting means. By using the
output of the second oxygen sensor 2, correction values for various
air-fuel ratio feedback factors are derived. At the air-fuel ratio
feedback control circuit B', as is understood from FIG. 13, the correction
values are used for forming corrected wave forms with which the basis
pulses are corrected.
FIG. 14 is a flowchart showing the outline of operation steps carried out
in the control unit.
That is, at Step 1, an engine speed "N" and an intake air amount "Q" are
read. At Step 2, a judgement as to whether or not the engine condition is
the certain condition permitted by the engine condition detecting means is
carried out. If Yes, that is, when the engine is under the certain
condition, the operation flow goes to Step 3. At this step, the output of
the second oxygen sensor 2 is read for a given time. Of course, the output
of the sensor 2 is converted to a digital form from an analogue form.
Then, at Step 4, a difference from a desired air-fuel ratio is calculated
and the air-fuel ratio feedback factor is corrected in accordance with the
difference. This step will be described in detail hereinafter. Then, at
Step 5, a judgement as to whether the output of the second oxygen sensor 2
is normal or not is carried out. If Yes, the work at this subroutine is
finished.
FIG. 15 shows the detail of Step 4 of the flowchart of FIG. 14.
As the air-fuel ratio feedback factors, proportional factors "P" and
integral factors "I" are commonly used. However, in the step 4 of FIG. 15,
proportional factors "P" are used, which are corrected in the following
manner.
As will become apparent as the description proceeds, in the step 4, a
judgement as to whether the output curve of the second oxygen sensor 2
intersects both or one of the slice levels SR and SL or fails to intersect
both of them is carried out.
At Step 41, the maximum and minimum values V-max and V-min of the output
"V" of the second oxygen sensor 2 are read. At Step 42, a judgement as to
whether V-max is greater than SR or not is carried out. If Yes, the
operation flow goes to Step 43 where a judgement as to whether V-min is
smaller than SL or not is carried out. If Yes, the operation flow goes to
Step 44 and it is judged that the air-fuel ratio of the air-fuel mixture
is desirable, that is, kept within the stoichiometric level. That is,
these steps show the condition of FIG. 9 wherein the output curve of the
second oxygen sensor intersects both the slice levels SR and SL. Thus, at
Step 45, the proportional factors PL' and PR' are set to the original
values PL and PR.
If No at Step 42, the operation flow goes to Step 46. At this step, a
judgement as to whether V-max is smaller than SL or not is carried out. If
Yes, the operation flow goes to Step 47 and it is judged that the air-fuel
ratio of the mixture is leaner than stoichiometric. These steps thus show
the condition of FIG. 11 wherein the output curve of the second oxygen
sensor 2 is located at the leaner side and fails to intersect both the
slice levels SL and SR. Thus, at Step 48, the proportional factor PR' is
determined to "PR (1+Kp.multidot..beta.)" to enrich the air-fuel mixture
fed to the engine causing the air-fuel ratio of the mixture to become
stoichiometric as soon as possible.
If No at Step 46, the operation flow goes to Step 49 and it is judged that
the air-fuel ratio of the mixture is slightly leaner than stoichiometric.
These steps thus show the condition of FIG. 10 wherein the output curve of
the second oxygen sensor 2 intersects only the slice level SL. Thus, at
Step 50, the proportional factor PR' is determined to "PR (1+Kp)" to
somewhat enrich the air-fuel mixture causing the air-fuel ratio of the
mixture to become stoichiometric soon.
If No at Step 43, the operation flow goes to Step 51. At this step, a
judgement as to whether V-min is greater than SR or not is carried out. If
Yes, the operation flow goes to Step 52 and it is judged that the air-fuel
ratio of the mixture is richer than stoichiometric. These steps show the
condition of FIG. 7 wherein the output curve of the second oxygen sensor 2
is located at the richer side and fails to intersect both the slice levels
SR and SL. Thus, at Step 53, the proportional factor PL' is determined to
"PL (1+Kp.multidot..beta.)" to lean the air-fuel mixture fed to the engine
causing the air-fuel ratio of the mixture to become stoichiometric as soon
as possible.
If No at Step 51, the operation flow goes to Step 54 and it is judged that
the air-fuel ratio of the mixture is somewhat richer than stoichiometric.
These steps thus show the condition of FIG. 8 wherein the output curve of
the second oxygen sensor 2 intersects only the slice level SR. Thus, at
Step 55, the proportional factor PL' is determined to PL(1+Kp) to somewhat
lean the air-fuel mixture fed to the engine causing the air-fuel ratio of
the mixture to become stoichiometric soon.
After the above-mentioned judgement is carried out, the corrected
proportional factors PL' and PR' are outputted at Step 56. The operation
flow then goes to Step 5 of the flowchart of FIG. 14.
The above-mentioned five judgements will be itemized in the following.
##EQU1##
As will be understood from the above description, in the present invention,
the leaner and richer air-fuel supply to the engine, which may be caused
by deterioration of the first oxygen sensor, is detected by the second
oxygen sensor. The air-fuel ratio feedback factors (viz., the proportional
factors PL and PR) are corrected in accordance with the information
signals issued from the second oxygen sensor and, the fuel injectors are
controlled in accordance with the corrected feedback factors. That is,
when the second oxygen sensor detects a richer or leaner condition of the
air-fuel mixture (which may be caused by deterioration of the first oxygen
sensor), the control unit issues command signals to the fuel injectors
until the mixture becomes to have a stoichiometric air-fuel ratio, that
is, until the second oxygen sensor exhibits an output characteristic which
is similar to that of the first oxygen sensor.
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